Conventionally, it has been known that a window panel mounted in a window opening of a vehicle is made of a resin material to reduce vehicle weight as described in PATENT DOCUMENT 1. Furthermore, it has been known that in order to reduce vehicle weight, a back door includes an outer panel which includes a window panel of a glass material and is made of a resin material, and an inner panel made of a resin material as described in PATENT DOCUMENT 2.